In offset lithographic printing the image to be printed is carried on a thin flexible planographic plate that is wrapped around and secured to the plate cylinder at both its leading and trailing ends. The leading end is normally bent rearwardly to form an acute angle between the bent portion and the remainder of the plate and then hooked onto an undercut edge formed in the plate cylinder. In the past, the trailing end of the plate was usually also bent and then locked into the same undercut area as the leading end, by some sort of lock-up mechanism.
For example, in U.S. Pat. No. 3,727,551 the trailing end of the plate is formed with a U-shaped bend and is engaged by a gripper carried within a rockable bar that is disposed within an undercut slot formed in the plate cylinder. In U.S. Pat. No. 3,757,690 the leading end is again hooked over an undercut edge while the trailing end is held within a rotatable rod in a groove in the plate cylinder by means of a separate clamping block. Additional examples of existing plate lock-up mechanisms can be found in U.S. Pat. No. 3,260,200; 4,347,788 and 4,495,865.
For a plate lock-up mechanism to be effective it must conform the plate to the surface of the plate cylinder solidly to lengthen the useful life of the plate and reduce plate cracking. Cracking would occur under conditions where the plate is not solidly against the cylinder and is thus able to flex during printing operations. One factor that has become important in modern lithographic printing is that of reducing the width of the non-print gap that exists between the two undercut edges on the plate cylinder. As printing costs have increased there has been increased emphasis on utilizing the maximum amount of newsprint. That is, when the non-print gap is wide a great deal of newsprint is effectively lost and this fact increases publishing costs.